12CaO • 7Al2O3 compound clathrating active oxygen species and method for preparing the same

ABSTRACT

The present invention provides a 12CaO.7Al 2 O 3  compound containing an O 2   −  ion radical and/or an O −  ion radical in a high concentration of 10 20  cm −3  or more. This compound can be used as an oxidization catalyst, antibacterial agent, ion conductor, or electrode material for solid-oxide fuel cells.

TECHNICAL FIELD

The present invention relates to a 12CaO.7Al₂O₃ compound which is an oxide clathrating an O₂ ⁻ ion radical and an O⁻ ion radical as active oxygen species in a high concentration (hereinafter, these two ion radicals are referred collectively to as “active oxygen species”). The present invention also relates to a method for producing such a compound and to the use thereof.

BACKGROUND ART

An O₂ ⁻ ion radical is known as one of active oxygen species which has a key role in oxidizing processes of various organic and inorganic materials. Extensive researches have heretofore been made on O₂ ⁻ absorbed on the solid surface of an oxide compound (J. H. Lunsford, Catal, Rev. 8, 135, 1973, M. Che and A. J. Tench, Adv. Catal, 32, 1, 1983). In most of such researches, high-energy gamma rays are irradiated onto the surface of an oxide compound to create O₂ ⁻ ion radicals thereon.

RO₂ (R:alkali metal) is known as a crystal including an O₂ ⁻ ion radical as a constituent anion. However, the related compounds are unavailable for a certain application such as oxidation catalysts or ionic conductors, because all of the compounds will be readily decomposed even at a temperature of 300° C. or less.

As compared to O₂ ⁻ ion radicals, O⁻ ion radicals have higher activity. Several articles have reported that a small amount of O⁻ ion radicals was included in alkali halide glasses, calcium-aluminosilicate glasses or the like (J. R. Bralsford et al., J. Chem. Physics, Vol. 49, pp 2237, 1968, H. Hosono et al., J. Am. Ceramic. Soc., 70, 867, 1987). However, there has not been known any crystal having an O⁻ ion radical as a constituent ion.

In 1970, H. B. Bartl et al. made a point that among sixty-six oxygens within a unit cell containing two molecules in a 12CaO.7Al₂O₃ crystal, so-called C12A7, two oxygens of them existed within a space of each cage structure in the crystal as “free oxygens” without residing in a network of the crystal (H. B. Bartl and T. Scheller, Neuses Jarhrb. Mineral., Monatsh, 1970, 547).

Based on an electron spin resonance analysis, Hosono, one of the inventors, et al. have discovered that about 1×10¹⁹ cm⁻³ of O₂ ⁻ was clathrated in a 12CaO.7Al₂O₃ crystal synthesized by reacting two raw materials, CaCO₃ and Al₂O₃ or CaCO₃ and Al (OH)₂, in a solid phase reaction at a temperature of 1200° C. in ambient atmosphere. They have proposed a model in which a part of free oxygens exists in each cage structure in the form of O₂ ⁻ (H. Hosono and Y. Abe, Inorg. Che. 26, 1193, 1987).

12CaO.7Al₂O₃ is inherently a stable oxide having a melting point of 1415° C. If a larger amount of active oxygen species can be clathrated in this oxide and then a reversible incorporation and release of oxygen can be achieved, the oxide would have a desirable availability for various purposes, such as oxidation catalysts or ionic conductors.

While one of the inventers, et al. have found out that O₂ ⁻ was clathrated in the 12CaO.7Al₂O₃ crystal, the concentration of O₂ ⁻ was a relatively low value of 10¹⁹ cm⁻³ and any O⁻ ion radical having higher activity has not been identified. Further, any effective technique for controlling the amount of O₂ ⁻ and releasing/incorporating it from/into the crystal reversibly has not been achieved.

For using such a compound as high-efficiency oxidation catalysts or antibacterial agents, it is required to clathrate the active oxygen species in a higher concentration and to provide a reversible function for releasing the clathrated active oxygen species and incorporating oxygen from outside. It is also necessary to establish a technique for quantitatively analyzing the concentration of the clathrated active oxygen species.

DISCLOSURE OF INVENTION

The inventers have discovered that a 12CaO.7Al₂O₃ compound clathrating active oxygen species in a high concentration of 10²⁰ cm⁻³ or more is obtained by preparing a raw material including calcium and aluminum mixed with each other in an atomic equivalent ratio of approximately 12: 14 and then reacting the raw material in a solid phase reaction at a controlled temperature under a controlled atmosphere. The present invention is directed to such a compound itself, a method for producing the same, a method for releasing clathrated ions, and the use of the compound.

More specifically, the present invention provides a 12CaO.7Al₂O₃ compound produced by preparing a raw material including calcium and aluminum mixed with each other in an atomic equivalent ratio of approximately 12:14, preferably a raw material including calcium carbonate and gamma-aluminum oxide mixed with each other in a molecular equivalent ratio of approximately 12:7, and then reacting the raw material in a solid phase reaction at a sintering temperature of 1200° C. or more, preferably 1300° C., under an atmosphere with an oxygen partial pressure of 10⁴ Pa or more and a water-vapor partial pressure of 10² Pa or less, preferably an oxygen partial pressure of 10⁵ Pa or more and a water-vapor partial pressure of 1 Pa or less. This compound can include 10²⁰ cm⁻³ or more of clathrated active oxygen species. The amount of the clathrated active oxygen species can be determined by an electron spin resonance analysis and a Raman spectrum analysis.

When the sintering atmosphere is arranged in an oxygen partial pressure less than of 10⁴ Pa and a water-vapor partial pressure of more than 10² Pa, the concentration of the clathrated active oxygen species will be less than 10²⁰ cm⁻³. Further, even under a dry oxidation atmosphere with an oxygen partial pressure of 10⁴ Pa or more and a water-vapor partial pressure of 10² Pa or less, when the sintering temperature is arranged in less than 1200° C., it will be difficult to synthesize the desired 12CaO.7Al₂O₃ compound. Conversely, when the sintering temperature exceeds 1415° C., the raw material will be undesirably molten. Thus, it will also be hard to obtain the desired 12CaO.7Al₂O₃ compound. In case of synthesizing the 12CaO.7Al₂O₃ compound through a solid phase reaction, the mixture of calcium carbonate and gamma-aluminum oxide is suitable for the raw material. However, any combination of calcium hydroxide or calcium oxide and aluminum hydroxide or one of various aluminum oxides (alpha, gamma or theta aluminum oxide) may be used as the raw material to synthesize the above compound.

An electron spin resonance (ESR) spectrum (at 77 K) of the 12CaO.7Al₂O₃ compound clathrating the active oxygen species is formed of a superposition of two spectrums; one defined by gx=2.00, gy=2.01 and gz=2.04, the other defined by gx=gy=2.05 and gz=2.00. These g values correspond to those of O₂ ⁻ ion radicals and O⁻ ion radicals in the solid, respectively. Thus, it can be concluded that O₂ ⁻ ion radicals and O⁻ ion radicals are clathrated in the 12CaO.7Al₂O₃ compound.

The absorption band shape in the ESR spectrum is symmetric at room temperature and becomes asymmetric at a low temperature of 77K. This indicates that O₂ ⁻ ion radicals and O⁻ ion radicals rotationally move within each cage structures at room temperature, while they are coupled electrostatically with and retained spatially by Ca²⁺ ions residing on each wall of the cage structures at low temperature. Each concentration of O₂ ⁻ ion radicals and O⁻ ion radicals can be quantitatively determined from the intensity of the absorption band.

In a Raman scattering spectrum of the above compound, a strong scattering peak is exhibited around 1130 cm⁻¹. This peak corresponds to the peak of O₂ ⁻ ion radicals which has been reported by K. Nakamoto et al. (K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compound, 1978, Wiley). Since there is a certain dependence between the ESR absorption band and the Raman scattering intensity, the intensity of clathrated O₂ ⁻ ion radicals can be quantitatively determined from the Raman scattering intensity. When the above compound is heated at 1200° C. or more under an oxygen partial pressure of 10⁴ Pa or less or a water-vapor partial pressure of 10² Pa or more, the active oxygen species or oxygen molecules will be released from the compound.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing the crystal structure of 12CaO.7Al₂O₃.

FIG. 2 is a graph showing an ESR spectrum of a compound obtained in an inventive example 1, at room temperature.

FIG. 3 is a graph showing an ESR spectrum (at 77K) of each compound obtained in the inventive example 1 and a comparative example 1.

FIG. 4 is a graph showing each Raman spectrum of the compounds obtained in the inventive example 1 and the comparative example 1.

FIG. 5 is a graph showing an analytical curve of temperature to release-gas intensity with a molecular mass/charge ratio=32 in each of the 12CaO.7Al₂O₃ compounds obtained in the inventive example 1 and comparative example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

As shown in FIG. 1, 12CaO.7Al₂O₃ has a cubic crystal system with the space group 143d, and 2 molecules of 12CaO.7Al₂O₃ exist in each unit lattice or unit cell of the crystal. 12CaO.7Al₂O₃ has a melting point of 1415° C. The crystal has a structure in which Ca²⁻ ions are located on a network structure formed of polymerized tetrahedrons of AlO₄. Each crystal lattice defines a space (cage) therewithin

That is, 2 (12CaO.7Al₂O₃)=Ca₂₄Al₂₈O₆₆=[Ca₂₄Al₂₈O₆₄]⁴⁺.2O²⁻. The O²⁻ is referred to as a free oxygen, which exists within the cage. Generally, the O²⁻ is coordinated by a cation all the time and thereby hardly to be free. However, in the 12CaO.7Al₂O₃ crystal, the O₂ ⁻ ion resides within the cage. Thus, the O²⁻ ion can be free without coupling with the cation. This state is referred to as “calthration”. In this state, the O²⁻ ion is chemically active as in the state when absorbed on a solid surface.

The O²⁻ ion clathrated by the cage is confined within the cage so as to prevent any reaction with the outside atmosphere. However, in a high temperature of 1200° C. or more, the size of the cage is thermally expanded. This allows oxygen molecules from the atmosphere to pass through the bottleneck of the cage. Consequently, the following reaction will be caused.

O²⁻ (within the cage)+O₂ (from the atmosphere)=O⁻ (within the cage)+O₂ ⁻ (within the cage)

Thus, a pair of O⁻ and O₂ ⁻ is generated from an oxygen ion O²⁻ residing in each of the unit cell as a result of the reaction with O₂ molecule. The 12CaO.7Al₂O₃ compound clathrating O⁻ and O₂ ⁻ in a high concentration can be represented by [Ca₂₄Al₂₈O₆₄]⁴⁺.m O₂ ⁻.m O⁻.(2−m) O²⁻, where m≦2, and the O₂ ⁻ and O²⁻ are clathrated within the cage.

By subjecting the compound to a heat treatment at a temperature of 1200° C. or more under an atmosphere with an oxygen partial pressure of 10⁴ Pa or less or a water-vapor partial pressure of 10² Pa or more, oxygen molecules from the atmosphere actively pass through the bottleneck of the cage. Thus, during the course of the transition of the concentration of the active oxygen ion radicals to a certain equilibrium concentration, the active oxygen ion radicals can be released from the cage to the atmosphere.

The O₂ ⁻ ion radicals clathrated in the 12CaO.7Al₂O₃ compound of the present invention can be quantitatively analyzed through the use of a scattering intensity caused by the O₂ ⁻ ion radicals around a Raman shift of 1128 cm⁻¹.

The O₂ ⁻ ion radicals and the O⁻ ion radicals each clathrated in the 12CaO.7Al₂O₃ compound can also be quantitatively analyzed through the use of two electron spin resonance absorption intensities; one defined by gx=2.00, gy=2.01 and gz=2.04, the other defined by gx=gy=2.05 and gz=2.00.

EXAMPLE

The present invention will be more specifically described in connection with inventive examples and a comparative example.

Inventive Example 1

A powder of raw material including calcium carbonate and gamma-alumina mixed with each other in a molecular equivalent ratio of 12:7 was sintered at 1300° C. under an atmosphere with an oxygen partial pressure of 1 Atm (10⁻¹ MPa) for two hours (sample 1). Through an X-ray diffraction analysis, the obtained sample 1 was verified as a 12CaO.7Al₂O₃ compound. An ESR spectrum of the obtained compound was measured at each of room temperature and 77K.

FIGS. 2 and 3 show ESR spectra of the obtained compound of the example 1 measured at room temperature and 77K, respectively. The wavelength of microwave used was 9.75 GHz. At room temperature, an absorption band having a symmetric shape with respect to a magnetic field intensity of 343 mT was observed. The g value was determined as 2.02. At 77K, an asymmetric absorption band was observed. Each of these absorption bands was formed of a superposition of one absorption band defined by gx=2.002, gy=2.009 and gz=2.073 arising from O₂ ⁻ ion radicals, and another absorption band defined by gx=gy=2.042 and gz=2.001 arising from O⁻ ion radicals. Each concentration of the O₂ ⁻ ion radicals and the O⁻ ion radicals is quantitatively determined as 1×10²¹ cm⁻³ from respective intensities of the absorption bands.

Comparative Example 1

A powder of raw material including calcium carbonate and gamma-alumina mixed with each other in a molecular equivalent ratio of 12:7 was sintered at 1300° C. in air (oxygen partial pressure of (2×10⁴ Pa, water-vapor partial pressure of more than 10² Pa)for two hours (sample 2 or comparative example 1). Through an X-ray diffraction analysis, the obtained sample 2 was verified as a 12CaO.7Al₂O₃ compound. FIG. 3 shows an ESR spectrum at 77K. In the comparative example 1 (sample 2), an absorption band is divided into three components, which are determined as gx=2.009, gy=2.002 and gz=2.073, respectively. This absorption band is caused by O₂ ⁻ ion radicals, and the concentration of the O₂ ⁻ ion radicals is quantitatively determined as 1×10¹⁹ cm⁻³.

A Raman spectrum was measured for each of the inventive example 1 (sample 1) and the comparative example 1 (sample 2). FIG. 4 shows the Raman spectra obtained from the inventive example 1 and the comparative example 1. In both the spectra, several scattering lines arising from [Ca₂₄Al₂₈O₆₄]⁴⁺ were observed in the energy range of 1000 cm⁻¹ or less. In the inventive example 1 (sample 1), an additional Raman peak arising from the O₂ ⁻ ion radicals was observed at a Raman shift of 1128 cm⁻¹.

Inventive Example 2

A powder of raw material including calcium carbonate and gamma-alumina mixed with each other in a molecular equivalent ratio of 12:7 was sintered at 1300° C. in room air for two hours (sample 1), and then additionally annealed at 1300° C. under an atmosphere with an oxygen partial pressure of 1 Atm (10⁻¹ MPa) for two hours (sample 3). Through an X-ray diffraction analysis, the obtained sample 3 was verified as a 12CaO.7Al₂O₃ compound. The concentration of O₂ ⁻ ion radicals included in the obtained 12CaO.7Al₂O₃ compound was quantitatively determined from ESR and Raman-scattering spectra. The determined amount of the clathrated O₂ ⁻ ion radicals was 1×10²¹ cm⁻³. From the ESR spectrum, it was also proved that O⁻ ion radicals were clathrated by a concentration of 1×10²¹ cm⁻³.

Inventive Example 3

The 12CaO.7Al₂O₃ compound obtained from the inventive example 1 was subjected to a heat treatment at 1300° C. in air (oxygen partial pressure of 2×10⁴ Pa, water-vapor partial pressure of more than 10² Pa) for two hours (sample 4). The concentration of O₂ ⁻ ion radicals included in the 12CaO.7Al₂O₃ compound after the heat treatment was quantitatively determined from ESR and Raman-scattering spectra. The determined amount of the clathrated O₂ ⁻ ion radicals was 1×10¹⁹ cm⁻³. It is proved that the active oxygen species have been reduced by 3.8−0.02=3.78 for each of the unit cell through the heat treatment. Most of these active oxygen species were released to the atmosphere.

The 12CaO.7Al₂O₃ compound obtained from the inventive example 1 was subjected to an analysis for determining a release-gas intensity with increasing temperature. The measurement was done in a vacuum atmosphere. It was confirmed that the obtained compound of the inventive example 1 exhibited a sharply rising peak with a molecular mass/charge ratio=32, and the active oxygen species were released from the compound. FIG. 5 shows an analytical curve of temperature to release-gas intensity with a molecular mass/charge ratio=32 in each of the 12CaO.7Al₂O₃ compounds obtained from the inventive example 1 and comparative example 1.

Concentrations of the active oxygens in the inventive examples and the comparative example will be shown in Table 1.

TABLE 1 concentration of concentration of Conditions of O₂ ⁻ ion radical O⁻ ion radical Burning and heating (cm⁻³) (per unit cell) (cm⁻³) (per unit cell) Inventive Example 1 In oxygen 1 Atm 1 × 10²¹/1.9 1 × 10²¹/1.9 1300° C., two hours Inventive Example 2 In air 1 × 10²¹/1.9 1 × 10²¹/1.9 1300° C., two hours + In oxygen 1 Atm 1300° C., two hours Inventive Example 3 In oxygen 1 Atm 1 × 10¹⁹/0.02 below sensitivity limits/ 1300° C., two hours + below sensitivity limits In air 1300° C., two hours Comparative Example 1 In air 1 × 10¹⁹/0.02 below sensitivity limits/ 1300° C., two hours below sensitivity limits

The amount of active oxygen species clathrated in each of the inventive example 1 (sample 1) and the inventive example 2 (sample 3) is 3.8 per unit cell. This value is almost equal to the theoretically calculated maximum total value of 2+2=4.

As shown the above examples, clathrated active oxygen species and incorporating oxygen from the atmosphere can be achieved by controlling the temperature and the oxygen partial pressure of the atmosphere during the sintering process of 12CaO.7Al₂O₃, and releasing of the clathrated active oxygen species can be achieved by the heat treatment of the obtained 12CaO.7Al₂O₃ compound under the controlled oxygen partial pressure of the atmosphere.

INDUSTRIAL APPLICABILITY

The present invention provides a 12CaO.7Al₂O₃ compound which clathrates active oxygen species in a high concentration of 10²⁰ cm⁻³ or more and allows the clathrated active oxygen species or oxygen ions to be released at a high temperature of 1250° C. or more. Thus, this compound can be used as an oxidization catalyst, for example, in order to oxidize organic materials. Further, the active oxygen species has an excellent antibacterial action as is generally known. Thus, the 12CaO.7Al₂O₃ compound clathrating a large amount of active oxygen species according to the present invention can also be used as an effective antibacterial agent.

Furthermore, the active oxygen species in the 12CaO.7Al₂O₃ compound clathrating a large amount of them can substantially freely move within the crystal structure. This opens the way for applying the 12CaO.7Al₂O₃ compound of the present invention to an ion conductor. Moreover, by combining the ion conductivity with the ability of oxidizing organic materials, this compound can be used as an electrode material for solid-oxide fuel cells. 

What is claimed is:
 1. A 12CaO.7Al₂O₃ compound comprising an O₂ ⁻ ion radical and/or an O⁻ ion radical serving as active oxygen species, said ion radical being clathrated in said compound in a concentration of 10²⁰ cm⁻³ or more.
 2. A method for producing a 12CaO.7Al₂O₃ compound comprising the steps of: preparing a raw material powder including calcium (Ca) and aluminum (Al) mixed with each other in an atomic equivalent ratio of 12:14; and reacting said raw material in a solid phase reaction at a sintering temperature ranging between 1200° C. or more and less than 1415° C., under a dry oxidization atmosphere with an oxygen partial pressure of 10⁴ Pa or more and a water-vapor partial pressure of 10² Pa or less.
 3. A method as defined in claim 2, wherein said raw material includes a calcium component selected from the group consisting of calcium carbonate, calcium hydroxide and calcium oxide, and an aluminum component selected from the group consisting of aluminum oxide and aluminum hydroxide.
 4. A method for releasing an active oxygen species clathrated in the 12CaO.7Al₂O₃ compound as defined in claim 1, characterized by subjecting said 12CaO.7Al₂O₃ compound to a heat treatment at a temperature of 1200° C. or more under an atmosphere with an oxygen partial pressure of less than 10⁴ Pa or a water-vapor partial pressure of more than 10² Pa.
 5. A method for quantitatively analyzing the O₂ ⁻ ion radical clathrated in the 12CaO.7Al₂O₃ compound as defined in claim 1, characterized in that an amount of said O₂ ⁻ ion radical is measured based on a scattering intensity arising from said O₂ ⁻ ion radical around a Raman shift of 1128 cm⁻¹.
 6. A method for quantitatively analyzing the O₂ ⁻ ion radical and O⁻ ion radical each clathrated in the 12CaO.7Al₂O₃ compound as defined in claim 1, characterized in that amounts of said O₂ ⁻ ion radical and said O⁻ ion radical are measured based on a first electron spin resonance absorption intensity defined by gx=2.00, gy=2.01 and gz=2.04, and a second electron spin resonance absorption intensity defined by gx=gy=2.05 and gz=2.00, respectively.
 7. An oxidization catalyst comprising a 12CaO.7Al₂O₃ compound including an O₂ ⁻ ion radical and/or an O⁻ ion radical serving as active oxygen species, said ion radical being clathrated in said compound in a concentration of 10²⁰ cm⁻³ or more.
 8. An antibacterial agent comprising a 12CaO.7Al₂O₃ compound including an O₂ ⁻ ion radical and/or an O⁻ ion radical serving as active oxygen species, said ion radical being clathrated in said compound in a concentration of 10²⁰ cm⁻³ or more.
 9. An ion conductor comprising a 12CaO.7Al₂O₃ compound including an O₂ ⁻ ion radical and/or an O⁻ ion radical serving as active oxygen species, said ion radical being clathrated in said compound in a concentration of 10²⁰ cm⁻³ or more.
 10. An electrode material for solid-oxide fuel cells, comprising a 12CaO.7Al₂O₃ compound including an O₂ ⁻ ion radical and/or an O⁻ ion radical serving as active oxygen species, said ion radical being clathrated in said compound in a concentration of 10²⁰ cm⁻³ or more.
 11. A compound, as defined in claim 1, comprising an O⁻ ion radical in a concentration of 10²⁰ cm⁻³ or more.
 12. An oxidization catalyst, as defined in claim 7, wherein said 12CaO.7Al₂O₃ compound includes an O⁻ ion radical in a concentration of 10²⁰ cm⁻³ or more.
 13. An antibacterial agent, as defined in claim 8, wherein said 12CaO.7Al₂O₃ compound includes an O⁻ ion radical in a concentration of 10²⁰ cm⁻³ or more.
 14. An ion conductor, as defined in claim 9, wherein said 12CaO.7Al₂O₃compound includes an O⁻ ion radical in a concentration of 10²⁰ cm⁻³ or more.
 15. An electrode material, as defined in claim 10, wherein said 12CaO.7Al₂O₃ compound includes an O⁻ ion radical in a concentration of 10²⁰ cm⁻³ or more.
 16. A compound, as defined in claim 1, wherein said ion radical is clathrated in said compound in a concentration of 10²⁰ cm⁻³ or more during the synthesis of said compound by sintering a raw material powder including calcium (Ca) and aluminum (Al) mixed with each other in a solid phase reaction. 